1. Field of the Invention
This invention relates to a method for tagging an oleaginous and petroliferous substance so that it subsequently may be identified as to source. More particularly, the invention relates to such a method wherein control of the so-tagged substances is temporarily lost, as when they are stolen, spilled, misplaced or injected into a subterranean reservoir.
2. Description of the Prior Art
Sometimes one has in his control a first oleaginous and petroliferous substance. This control can then be lost, either deliberately, as when an oil is injected into a subterranean reservoir in an enhanced oil recovery process or in an oil disposal process, or inadvertently, as when an oil is spilled or stolen. Subsequently, there can come within one's control a second oleaginous substance which is suspected either to be or to contain a portion of the first oleaginous substance. This second oleaginous substance may be oil produced from a subterranean reservoir, discovered under circumstances arousing suspicion that it may be oil previously stolen or spilled, or encountered in any of a number of other circumstances.
Such oleaginous substances are difficult to identify. Some oleaginous and petroliferous substances, such as crude oil, have such a complex structure that their compositions are difficult to define and distinguish from each other. Other such substances, such as fractions of refined crude oil, have compositions so much alike that one sample from one source is difficult to distinguish from another sample from a different source. In either of the above instances, tracing a given sample of oil whose origin is unknown back to its source can prove onerous.
One instance in which control of an oil is lost is when the oil is injected into a subterranean petroleum-bearing reservoir. A typical reservoir is a stratum of rock containing tiny interconnected pore spaces which are saturated with oil, water and/or gas. When one or more wells are drilled into such a reservoir, fluids can be introduced into or withdrawn from the reservoir. This flow of fluid causes the indigenous reservoir fluids to migrate through the reservoir. Sometimes, as in enhanced oil recovery processes, plugging processes or waste disposal processes, fluids are injected into the reservoir via one well and simultaneously or subsequently fluids are produced from the reservoir via the same or a different well. It is often desired to determine whether the fluids produced contain any of the fluids which were injected. Such information is important in determining the permeability and porosity of the reservoir and the fluid flow characteristics through the reservoir.
It is known to add a tracer material to an injected fluid and to examine a produced fluid for the presence of the tracer material. Numerous materials have been suggested as tracer materials including radioactive materials, such as Iodine.sup.131, and other chemical compounds which are not commonly present in reservoir fluids in significant concentrations and are therefore easily detectable in small concentrations by conventional analytical techniques.
Most fluids injected into reservoirs during the above-described processes have been aqueous base fluids. Hence, the tracers employed have been water-soluble materials. However, it is sometimes desired to inject an oil into a reservoir and trace its flow through the reservoir. An oil or oil base enhanced oil recovery fluid has certain advantages over water base fluids used for this purpose. An oil base fluid is generally more compatible with reservoir oil and rock than is a water base fluid. Reservoirs sometimes contain clays which are substantially unaffected by oil but which swell when contacted by water. Such swelling can sharply decrease the permeability of a reservoir. In studying the flow characteristics through a reservoir, it is sometimes desired to inject an oil containing a tracer material into the reservoir via one well, produce fluids from the reservoir via one or more offset wells and examine the produced fluid to determine if any of the injected oil is present. In disposing of an oil or an oil base fluid in a reservoir, it is convenient to include in the fluid being disposed of an oil-soluble tracer material so that fluid movement through the reservoir can be monitored.
Therefore, it is a principal object of this invention to provide a method for tracing an oil.
It is a further object to provide such a method wherein an oil-soluble tracer material is added to a first oil, control of the first oil is lost, and subsequently a second oil is located and analyzed for the presence of the tracer added to the first oil.
It is a still further object to provide such a method for investigating the dynamic conditions of fluid flow through a subterranean reservoir penetrated by one or more wells.
It is another object to provide such a method wherein a fluid is injected into the reservoir and its flow through the reservoir is monitored.
It is yet another object to provide such a method wherein an oil or an oil base fluid containing a tracer material is injected into the reservoir via one well, reservoir fluids are produced via the same or a different well and the produced fluids are analyzed for the presence of the tracer material.
Other objects, advantages and features of this invention will become apparent to those skilled in the art from the following description and appended claims.